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Saturday, September 24, 2016

Q. Can you explain the origins of wavetable, S&S and vector synthesis?

By Steve Howell

The PPG Wave wavetable synthesizer. This one belongs to synth programmer, engineer and producer Nigel Bates.

I keep reading about different types of synthesis like 'wavetable',
'S&S' and 'vector' but I don't know what they are. I've looked
around the net for information but either the descriptions are very
simplistic or they're too technical. Could someone at SOS please explain the origins of these techniques?

Michael Cullen

SOS contributor Steve Howell replies: 'Wavetable
synthesis' is actually quite easy to understand. In the early days of
synthesis, (analogue) oscillators provided a limited range of waveforms,
such as sine, triangle, sawtooth and square/pulse, normally selected
from a rotary switch. This gave the user a surprisingly wide range of
basic sounds to play with, especially when different waveforms were
combined in various ways.
However, in the late '70s, Wolfgang Palm [of PPG] used 'wavetable'
digital oscillators in his innovative PPG Wave synths. Instead of having
just three or four waveforms, a wavetable oscillator can have many more
— say, 64 — because they are digitally created and stored in a 'look-up
table' that is accessed by a front-panel control. As you move the
control, so you hear the different waveforms as they are read out of the
table — the control is effectively a 64-way switch. If nothing else,
this gives a wide palette of waveforms to use as the basis of your
sounds. However, the waveform-selection control is not a physical switch
as such, but a continuously variable control implemented in software.
The advantage this has (apart from the 60 extra waveforms!) is that it
is also possible to use LFOs or envelopes or MIDI controllers to step
through these waveforms.
Now, if the waveforms are sensibly arranged, we can begin to create
harmonic movement in the sound. For example, if Wave 1 is a sine wave
and Wave 64 is a bright square wave with Waves 2 to 63 gradually getting
brighter as extra harmonics are added in each step of the wavetable, as
you move through the wavetable, you approach something not unlike a
traditional filter sweep. However, one disadvantage to this (but
something that characterised the PPG) is that the sweep will not be
smooth — the waveforms will step in audible increments.
Each oscillator in the PPG, however, didn't just have one wavetable —
there were 32 wavetables, each with 64 waveforms! Many were simple
harmonic progressions as described above; others were rudimentary
attempts at multisampling, whilst others attempted to emulate oscillator
sync sweeps and PWM (pulse-width modulation) effects. Because the
wavetable sweeping was so audibly stepped, the latter two weren't
entirely convincing emulations, though they had a character all their
own nonetheless.
Where things begin to get interesting, however, is when the waveforms
in the wavetable are disparate and harmonically unrelated, as the tonal
changes become random and unpredictable. For many, this feature of
wavetable synthesis was unusable, but some creative individuals like Tom
Dolby exploited it to create unique and distinctive sounds, as can be
heard on his 1982 album The Golden Age Of Wireless.
The PPG had something of a trump up its sleeve, however — totally
analogue filters! Using these, it was possible to smooth out the
wavetable sweeps. Another endearing quality of the PPG was its
low-resolution digital circuitry, which exhibited aliasing at extreme
frequencies that added a certain 'gritty' quality to the sound. Later
manifestations of the PPG (in Waldorf products) were of a higher quality
and offered smooth wavetable sweeping. But while they sounded better,
they lacked that (arguably) essential 'lo-fi' character.
Other synths have employed wavetable synthesis in one guise or
another since then and there are several software synths available today
which incorporate wavetable synthesis capabilities.The massively influential Korg M1 really put S&S synthesis on the map.

'S&S' is an abbreviation for 'samples and synthesis' and
refers to the new breed of synth that appeared with the introduction of
the seminal Roland D50 in 1987. Whereas synths prior to this used
analogue or digital oscillators to create sound, samplers were now in
the ascendent, with the introduction of affordable sampling products
such as the Ensoniq Mirage, the Emu Emax and the Akai S900. These
allowed almost any sound to be sampled and mangled but they had one
inconvenience — the samples took time to load and were inconveniently
stored on floppy disks. Roland could see that by using short samples as
the basic sound sources, and storing them in ROM for instant recall,
they could make the same type of sound as a sampler but with no tedious
load times. However, they also retained many of their previous
synthesizers' functions such as multi-mode filters, envelopes, LFOs and
so on. To all intents and purposes, the D50 'felt' like a synth but
sounded like a sampler. Furthermore, to smooth out any inadequacies in
the very short samples such as clicky and/or obvious loops, the D50 also
had chorus and reverb which 'smudged' these artifacts quite
effectively.
And so a legend — and a new synthesis method — was
born! Roland called it 'LA (linear arithmetic) synthesis'. In truth, it
was a simple layering method where up to four samples could be stacked
to create more complex sounds. Because of memory constraints (ROM/RAM
was very expensive at the time), Roland had to use very short samples,
and there were two categories of sample on the D50 — short, unlooped
samples (such as flute 'chiff' or guitar 'pluck') and short sustaining
loops. By combining and layering, for example, a flute 'chiff' with a
sustained flute loop sample, you could (in theory) create a realistic
flute sound. In practice, it didn't quite work out like that, but this
layering technique also gave the instrument a new palette of sounds to
work with and it was possible to layer, say, the attack of a piano with
the sustain of a violin. With the wealth of synthesis functions
available to process the samples, this allowed the user to create
interesting hybrid sounds.
Korg took this concept to a new level a
year or so later when they released their M1, another legend in modern
music technology. Although similar concepts were involved, the M1 used
longer, more complete samples which, in conjunction with typical synth
facilities, blurred the distinction between synth and sampler and
arguably heralded the beginning of the slow, gradual demise of the
hardware sampler! However, as well as advancing S&S, they also added
a very functional multitrack sequencer and good quality
multi-effects so that (maybe for the first time) it was possible to
create complete works on a single, relatively affordable keyboard. And
so the 'S&S workstation' was born. I think it's fair to say that
most modern synths owe something to the Korg M1 in one or another aspect
of their design.The ill-fated Sequential Circuits Prophet VS introduced vector synthesis to the world.These
days, many synths and keyboards routinely use these same basic
principles, but memory is now far more affordable and so it is possible
to have many more (and considerably more detailed) multisamples in the
onboard ROM. Whereas early S&S synths boasted around 4MB of onboard
ROM, figures of 60MB or more are bandied about today. That said, many of
the same techniques used for optimising samples and squeezing as many
into ROM as possible are still used today.
'Vector synthesis' is a
slightly different (but related) technique. First pioneered by Dave
Smith in his Prophet VS, vector synthesis typically uses four
oscillators which the user can 'morph' smoothly between, using real-time
controllers such as a joystick or automated controllers such as LFOs
and/or envelope generators. As the joystick is moved, so the balance of
the four oscillators changes and, depending on the nature of the source
waveforms, many interesting, evolving sounds can be created. But the
Prophet VS was ill-fated — Sequential Circuits were in financial trouble
and the company soon went to the wall. However, the concept lived on in
the Korg Wavestation, which was a joint venture between a
post-Sequential Smith and Korg. The Wavestation had a significant
advantage over the VS in that it used multisampled waveforms, allowing
more complex building blocks to be used — in many ways, it was a hybrid
S&S and vector synth. As well as extensive synth facilities
(filters, multi-stage envelopes and so on), it also had comprehensive
multi-effects and other facilities (not least of which was 'Wave
Sequencing') that made the Wavestation a programmer's dream, and a
casual user's nightmare! Indeed, they are still a staple component in
many players' keyboard rigs today. The Wavestation was discontinued many
years ago (though it's been resurrected in Korg's Legacy Collection software), but vector synthesis lives on in Dave Smith's Evolver range of keyboards.
If you're looking for further information on synthesis out there on the web, I can suggest two sections of the Sound On Sound
web site worth investigating. Paul Wiffen's 12-part Synth School
series, which appeared in the magazine between June 1997 and October
1998, is a good introduction to the basics of synthesis in its various
forms. If you enter "synth school" into the search engine at www.soundonsound.com,
you'll find it. Judging by your comments, you may find some of Gordon
Reid's long-running Synth Secrets series too technical, but it's
nevertheless worth a mention as it covered so much ground in its
five-year tenure. To make this vast amount of material a little easier
to navigate, we have created a special page with links to all of the
Synth Secrets articles: www.soundonsound.com/sos/allsynthsecrets.htm.